The problem is that most mutations are delaterious or neutral. Advantageous mutations are rare and take time to fixate in a population (function of conveyed advantage, generation time and lifetime fucundity) and generally only convey a small relative advantage. Delaterious mutations on the other hand are generally 100% deadly and (this is key) advantageous mutations don't help you defend against mutations. Delterious mutations are essentially random death from on high. Evolution requires a low enough deleterious mutation rate that advantageous mutations have "time" to become relatively widespread in a population relatively quickly relative to the delaterious mutation rate. If the deleterious mutation rate is too high, advantageous mutations can't fixate before they are destroyed.
Let me try an example. Assume 500 out of 1000 mutations are delaterious. 499 are neutral and 1 is advantageous. (rather optomistic). Thus, the chances a mutation will adversely impact you is 50%. Now assume the birth mutation rate is 100%. Namely, all births mutate. That's a pretty high mutation rate. This means 50% of all offspring will have lower fitness, 49.9% will have the same fitness and one out of 1000 will have greater fitness than it's parent.
With a fixed population (like in our sims) average lifetime fucundity is 1 given asexually reproducing organisms (it has to be or the population would increase or decrease over time) but that is an average. Some die without giving birth. Some give birth more than once but one or more offspring die before reproducing. On average, one offspring for each parent survives to reproduce. At 50% deleterious mutations, if we assumed mutations were the only thing impacting reproductive sucess, then on average, every orgamism has two offspring, one of which dies at boirth and one of which survives to reproduce. On average. If we assume preditors and accidents and just being plain unlucky, the ratio is higher. Lets assume on average, a bot has 4 offspring in it's life, one of which survives to reproduce. Half are killed at birth due to deleterious mutations and one of the four is killed by something else before he can reproduce. Lets also assume an advantagous mutation improves a bot's chances of surviving to reproduce by 10% (also very opotomistic) but for simplicity sake, if a bot lives that long, he reproduces 4 times. Namely, an organism with an advantageous mutation will have on average, a 10% better chance of surviving to reproduce relative to those without it.
What is the probability that when advantageous mutation occurs, it can survive long enough to fixate in the population? Given the high 100% mutation rate, the chances are bascially 0. The chances are 55% an organism born with an advantageous mutation will survice to reproduce (by definition, it was born without a delterious mutation, so it's one of the two offsrping that on average, are neutral or advantageous and since it is in fact one of the 1 in 1000 with an advantageous mutation, it has a 10% better chance of surviving than it's neutral sibling, but only 10%. It can still get eaten, fall in a hole, starve, etc.). But of the 4 offspring it will have on average if it survies to reproduce, half will die at birth due to delterious mutations. Their inherited advantage helps them there not at all. Of the two that don't die at birth, the chances of each surviving to reproduce is similarly 0.55. Thus, the chances an advantageous mutation survives to the third generation is the probability of the first surviving multiplied by the probabilitiy of at least one of the second surviving: (.55) * (.55 + (.55/2)) = 45%. Already less than 50/50. This is because both things must happen for the mutation to still be in the population at this stage: the original bot with the mutation must have survived to reproduce AND one or both of it's neutral offspring must also have survived. The chances of surviving to the 4th generation are lower still and so on. I may not have the math exactly right, but you get the idea. The high mutation rate prevents even highly advantageous mutations from surviving long enough to fixate. It's like trying to survive in a nuclear reactor.
So, bottom line, you need a low enough mutation rate so that when an advantageous mutation does happen, it can spread in the population before delterious mutations wipe it out...